In recent years, having the objective of minimizing the emission of harmful gases to the environment, and of particulate materials and/or other GHGs (greenhouse gases), a series of technologies has been incorporated into engines.
The reduction in emissions of gases is related, inter alia, to the increase in the thermal performance of the engine and, consequently, to the reduction in the specific consumption of fuel. As a consequence, engines are developing greater power per volume of displacement of the piston in the cylinder. The combustion engines are working under greater mechanical stresses, at higher rotation and higher combustion temperature. In this manner, the components thereof must be dimensioned to support these harsher operating conditions with the objective of ensuring both the reliability of the assembly and the maintenance of the working life expected. This greater operational stress is translated, likewise, into a greater stress experienced by the components, inter alia the piston and the rings associated with the piston.
With greater rates of compression, combustion pressure, temperature and rotation, the rings exert a greater pressure upon the piston and upon the cylinder walls, leading to greater wear and fatigue of the rings.
The aspects of wear and fatigue directly affect the durability of the piston rings, being connected to the maintenance of the performance of sealing and controlling the oil film within the design limits of the engine. These limits are related to emissions of pollutants, fuel consumption, and lubricating oil consumption, topics associated with public health. Furthermore, the lower fatigue strength may lead to the rupture of the ring and, consequently, to the seizure of the engine. In addition to the economic importance thereof, the seizure of the engine is a fundamental item in the safety of the vehicle.
The materials commonly utilized for the manufacture of piston rings are cast irons and high chromium martensitic steels. The process for the manufacture of the rings of cast iron may be summarized in the following stages: (1) melting of the alloy and correction of composition; (2) pouring into green sand molds or centrifugation; (3) heat treatment of the rings obtained; (4) machining to define the final dimensions; (5) treatments of surfaces such as nitridation or coatings to obtain a surface having high hardness. There is a technical limitation related to the mechanical strength for the utilization of rings of cast iron in engines having a high load or in engines requiring small cross sectional dimensions of the rings.
For the applications wherein the loads are high, the rings of cast iron are replaced by rings of high chromium martensitic steels. These are obtained by mechanical forming of drawn wires, for example as revealed in the document US 20070187002 (‘Piston ring excellent in resistance to scuffing, cracking and fatigue resistances, and method for producing the same, and combination of piston ring and cylinder block’), and they pass through the stages (4) and (5) aforedescribed. Nevertheless, there are limitations on the dimensions of the rings which may be produced by this process of manufacture, in addition to technical disadvantages of geometric shape definition, demanding a more complex process of manufacture and finishing. In order to fulfil the diverse dimensions, there is a requirement of maintenance of a large stock of material, increasing the costs involved.
A relevant technical aspect connected with the process of obtainment of rings by mechanical forming of drawn wires of high chromium steel is the presence of microcracks arising from the decohesion between carbides and matrix caused by the accumulation of cross sectional reductions in the stages of rolling and drawing, notably in the regions of the microstructure wherein agglomeration of carbides occurs. These problems are resolved by rings of high chromium steels produced by casting processes similar to those utilized for the production of rings of cast iron.
In the case of the cast rings of high chromium steel, the microstructure of the material obtained, in conformity with the description of the document US 20120090462, presents coarse M7C3 eutectic carbides formed during the solidification. FIG. 1 shows the microstructure of a material obtained in conformity with the description of the example of embodiment of the document US 20120090462 in the states as-cast and following tempering (final stage of the development of the microstructure), it being possible to observe that the coarse M7C3 eutectic carbides precipitated in a continuous manner in the interdendritic regions do not change with the application of the cycles of heat treatment up to tempering. Continuous networks of eutectic carbides are preferential sites for the nucleation and the propagation of fatigue cracks and, consequently, the presence thereof implies prejudicing the performance of piston rings.
The realization of surface treatments to obtain high hardness, low wear coefficient with the liner, and high wear resistance is usual for piston rings. Nitridation is the process of treatment of surfaces commonly utilized on piston rings, being realized with gas, plasma or in a salt bath. Optionally, an anti-wear coating may be applied upon the face of contact with the cylinder utilizing materials and processes of deposition forming part of the state of the art.